How do possesives work in Spanish?

The Spanish possesives agree in number with the nouns that they modify. A singular noun will have a singular possesive. A plural noun will have a plural possesive.

Examples:
'Mi perro es negro.' (= My dog is black).
'Mis perros son amistosos.' (= My dogs are friendly).

For the 'nosotros' and 'vosotros' forms, the possesive alsoagree with gender: masculine forms are 'nuestro', 'vuestro', 'nuestros' y 'vuestros'; feminine forms are 'nuestra', 'vuestra', 'nuestras' y 'vuestras'.

Examples:
'No me gusta vuestro vino.' (= I don't like your wine).
'Nuestras papas fritas son saladas.' (= Our French fries are salty.)

What are the Spanish possesives?

subject	corresponding possesives	examples
yo	mi mis	mi casa mis casas
tú	tu tus	tu casa tus casas
él/ella/usted	su sus	su casa sus casas
nosotros	nuestro nuestra nuestros nuestras	nuestro perro nuestra casa nuestros perros nuestras casas
vosotros/vosotras	vuestro vuestra vuestros vuestras	vuestro perro vuestra casa vuestros perros vuestras casas
ellos/ellas/ustedes	su sus	su casa sus casas

Note:

The possesive agree with what is being possessed, not with who is doing the possesing. So, when two people own one thing (like a house), you use the singular possesive because what they own is singular.

Example:
'Ricardo y Ana viven en su casa'. (= Ricardo and Ana live in their house.)

Likewise, when one person owns many things, you use the plural possesive because what that person owns is plural.

Example:
'Yo quiero mis zapatos' (= I want my shoes).